The Evolution of the Ear

Some eighteen days into the development of a human embryo, even before
the brain has become a complete organ, a group of surface cells on
each
side of the head begins to dimple. Each one of these groups forms a
hollow
sphere of cells in the shape of a bubble as it moves into the
substance
of the head. The cells then squirm and contort to create the various
parts
of the ear.

The human ear is a sensory organ both of hearing and balance. The
balance
apparatus appears to have evolved prior to the hearing mechanism.
Early
developing vertebrates such as fish have organs of balance, but no
cochlea.

Embedded under the skin of a fish, along the length of its head and
body,
is a series of depressions or grooves known as the
'lateral-line'. Groups
of hair cells just beneath the grooves detect differences in water
pressure,
which allows the fish to adjust to variations in currents and eddies,
and
to warn against the proximity of other fish, including predators. At
the
beginning of life in the oceans, even the most primitive fish
possessed
this simple sense organ.

Gradually, the grooves in the head evolved into the structure of the
inner
ear found in all vertebrates, including humans. It is easy to imagine
that
nerve cells in the inner ear are adaptations of earlier hair cells
sensitive
to the motions of liquid.

During the course of evolution, as fish became more amphibious, and
finally
developed into pure land animals, they required a new kind of sense
organ
which could detect slight differences in air pressure as a means of
increasing
their survival advantages, such as recognizing food, danger, friends,
and
enemies.

It is likely that the middle ear and the Eustachian tube evolved from
the
respiratory apparatus of the fish, while various inner ear structures
were
developed from parts of the fish jaw.

Eventually, the inner ear began to change and develop, in combination
with
new environmental pressures. It is probable that a small region of
the inner
ear partially responsible for balance evolved into the membrane of
the oval
window, which was flexible enough to transmit changes in air pressure
to
the fluid in the inner ear. At the same time, the inner ear was
increasing
in size and complexity. In amphibians, a small bulge appeared in the
vestibular
region of the ear, and as evolution proceeded, the bulge eventually
developed
into the spiraled cochlea which today forms the hearing mechanism of
the
inner ear of all vertebrates.

The range of frequencies which the ear is able to detect and analyze is
likely the result of evolutionary pressure to decode complex speech
sounds.
Similarly, the amplitude range probably evolved in response to the
loudest
sounds in the natural environment. This would include the cracks and
booms
of a thunderstorm at close range, as well as the loud roar of
predatory
animals. These sounds tend to rise slowly rather than abruptly. And
this
may explain why the ear has no defense against extremely loud sounds
which
occur suddenly, without warning.

The modern cochlea, with its power to recognize the separate vibrations
of each sound, has an obvious survival advantage. Since any sound
which
has been analyzed and transmitted to the brain can be remembered,
those
sounds which are associated with danger or with a promise of
fulfillment
can be acted upon immediately when heard again.